As a receiving system in a radio communication apparatus, there are a super-heterodyne system, a direct conversion system, a Low-IF system, or the like. While the receiving system which is most dominant at present is the super-heterodyne system, the direct conversion system and the Low-IF system have been recently noticed.
FIG. 16 is a block diagram illustrating a common direct conversion receiver.
A direct conversion receiver is one which performs frequency conversion from the RF band to DC without mediating the IF band, and this is operated as follows. A high frequency signal inputted from an antenna 201 is filtered by a band-pass filter (hereinafter, referred to as BPF) 202, and is subjected to signal amplification by a high frequency amplifier (hereinafter, referred to as an LNA) 203, and the resulted high frequency signal is divided into two paths to be inputted to mixers 204a and 204b. A 90° phase shifter 207 produces LO signals having a phase difference of 90° with each other from the signal from the PLL 208, and frequency conversions are carried out in the mixers 204a, 204b using these LO signals. Then, the signals from the mixers 204a, 204b are made pass through the low pass filters 205a, 205b, and they are amplified by VGA 206a, 206b up to desired amplitudes, to be outputted as output signals.
Since the direct conversion system carries out frequency conversion from RF band to DC without mediating IF band using a single mixer, the system construction thereof is simplified. In addition, since there arises no image crosstalk which causes a problem in the super heterodyne system, the number of BPFs can be greatly reduced. Accordingly, it has quite high contributions to cost reduction.
While the direct conversion system is an ideal receiving system as described above, it has following problems.
The problem is that since the frequency base band thereof is DC, it is likely to be affected by flicker noises with relative to the super heterodyne system. In particular, when MOS devices which are generally considered as having flicker noises of 100 to 1000 times with relative to high frequency devices such as bipolar transistors, it may become quite a no little problem (for example, refer to Nonpatent Document 1).
To specifically show this problem, a noise figure of a system in which a LNA 203 and a mixer 204a are connected in cascade connection as shown in FIG. 17 will be described.
When the gain Glna and the noise figure NFlna of the LNA 203 simple are constant as Glna=20 dB and NFlna=5 dB, respectively, and the noise figure NFmix of the mixer 204a simple has flicker noise characteristics which are in inverse proportion to the frequency at low frequency as shown in FIG. 18 (NFmix=15 dB@10 MHz, NFmix=45 dB@1 kHz), the noise figure of the entire system is NFall=5.4 dB at 10 MHz, while NFall=25 dB at 1 kHz from Friis equation. That is, the NFall when the IF signal frequency is high is approximately determined by NFlna, while the NFall when the IF signal frequency is low is approximately decided by NFmix-Glna, strongly depending on NFmix.
Accordingly, in a receiver that employs a direct conversion system or a Low-IF system, the reception sensitivity of the entire system is greatly deteriorated due to the low-frequency noises of the mixer.
The low frequency noise characteristics of the mixer will be described more concretely. Since while there are a single balanced mixer and a double balanced mixer as main stream mixers at present, these may not have large differences in their operations, a description is given of a signal balanced mixer here as one representing the both.
FIG. 19 shows a circuit diagram of a conventional mixer circuit. The basic construction of a mixer core portion thereof is a signal balanced mixer. In addition, numeral 11 denotes an RF transistor, numerals 21 and 22 denote first and second LO (local) transistors, numerals 33 and 34 denote first and second IF output terminals, numerals 31 and 32 denote first and second load resistors, numeral 50 denotes an RF signal supplier, numeral 60 denotes an LO signal supplier, VDD denotes a power supply, and GND denotes the ground.
The RF signal supplier 50 normally comprises an antenna or the like, and it corresponds, for example, to antenna 201, BPF 202, and LNA 203 shown in FIG. 16. The LO signal supplier 60 is normally a PLL or the like, and it corresponds, for example, to PLL 208 and 90° phase shifter 207 shown in FIG. 16.
Initially, the fundamental operation of the mixer circuit will be described.
An RF signal supplied from the RF signal supplier 50 is inputted to the RF transistor 11, and is converted from a voltage signal to a current signal.
On the other hand, differential LO signals which are supplied from the LO signal supplier 60 are inputted to the first and second LO transistors 21 and 22, respectively, and the first and second LO transistors 21 and 22 repeat switching operations at the frequency of the LO signals.
When the RF signal under being converted into a current signal is inputted to the first and second LO transistors 21 and 22 which are performing the switching operation, the RF signal and the LO signal are multiplied. Thereby, the RF signal is subjected to frequency conversion to be an IF signal, and the IF signal is subjected to voltage conversion by the first and second load resistors 31 and 32, thereby voltage IF signals are obtained through the first and second IF output terminals 33 and 34.
Next, the noise characteristics of the conventional mixer circuit will be described.
FIG. 20 shows the noise occupancy ratio of the flicker noises of the first and second LO transistors 21 and 22 in the IF frequency at the first and second IF output terminals 33 and 34. As shown in FIG. 20, more than 70% of the output noises at the frequency of 1 MHz or lower are flicker noises of the first and second LO transistors 21 and 22. Therefore, it is quite effective to suppress the flicker noises of the first and second LO transistors 21 and 22 in order to improve the noise characteristics at low frequency.
Here, the flicker noise occupancy ratio characteristics of the first and second LO transistors 21 and 22 shown in FIG. 20, and graphs of the noise figure characteristics which are shown hereinafter, are all the results of simulations employing the standard SPICE (Simulation Program with Integrated Circuit Emphasis).
Next, the flicker noises of the first and second LO transistors 21 and 22 in the conventional mixer circuit will be described more quantitatively.
First of all, it is known that the noise Vn2 at the gate terminals of the first and second LO transistors 21 and 22 are given by Formula 1.
                              Vn          2                =                  kf                      Cox            ·            W            ·            L            ·            f                                              [                  Formula          ⁢                                          ⁢          1                ]            
where Cox, W, and L denote gate oxide film capacitance, channel width, and channel length of the first and second LO transistors 21 and 22, respectively, f denotes the frequency, and kf denotes flicker coefficient.
The Vn is converted into a current by a trans-conductance gmLO of the first and second LO transistors 21 and 22, and further is converted into a voltage by the first and second load resistors 31 and 32 to appear at the first and second IF output terminals 33 and 34. Accordingly, output noise Vno2 of the first and second LO transistors 21 and 22 which appear at the first and second IF output terminals 33 and 34, are represented by Formula 2.
                              Vno          2                =                  α          ·                      gmLO            2                    ·                      R            2                    ·                      kf                          Cox              ·              W              ·              L              ·              f                                                          [                  Formula          ⁢                                          ⁢          2                ]            
where R denotes the resistance of the first and second load resistors 31 and 32, and α denotes a constant.
Accordingly, the input conversion noise Vni2 is obtained by dividing the output noise Vno2 by the power gain β·gmRF2·R2, to be represented by Formula 3.
                              Vni          2                =                              α            β                    ·                                    gmLO              2                                      gmRF              2                                ·                      kf                          Cox              ·              W              ·              L              ·              f                                                          [                  Formula          ⁢                                          ⁢          3                ]            
where gmRF denotes a trans-conductance of the RF transistor 11, and β denotes a constant.
Further, when the input conversion noise Vni2 is expressed by the noise figure NF of a 50Ω system, it is presented by Formula 4.
                    NF        =                  10          ·                      log            ⁡                          (                                                γ                                      50                    ·                    k                    ·                    T                                                  ·                                                      gmLO                    2                                                        gmRF                    2                                                  ·                                  kf                                      Cox                    ·                    W                    ·                    L                    ·                    f                                                              )                                                          [                  Formula          ⁢                                          ⁢          4                ]            
where k denotes the Boltzmann constant, T denotes an absolute temperature, and γ=α/β.
As conventional techniques, there are following measures as means for improving the NF characteristics at low frequency of the mixer circuit.
The first conventional technique is to increase the transistor sizes of the first and second transistors 21 and 22. The flicker noises are in inverse proportion to the LW products of the first and second LO transistors 21 and 22, as shown by Formula 1. Therefore, by increasing the transistor sizes of the first and second LO transistors 21 and 22, i.e., the LW products, it is possible to improve the NF characteristics according to Formula 4.
The second conventional technique is to increase the gain of the mixer circuit. In order to do so, the trans-conductance gmRF of the RF transistor 11 is to be increased, and this can be realized by increasing the W/L ratio of the RF transistor 11, or by increasing the bias current of the RF transistor 11. Thereby, the input conversion noise can be reduced, and consequently, the NF characteristics be improved according to Formula 4.
The third conventional technique is to optimize the sizes of the first and second load resistors 31 and 32. When the output noises at low frequency are dominated by flicker noises and resistor thermal noises of the first and second load resistors 31 and 32, the ratio between the resistor thermal noises and the flicker noises can be optimized by adjusting the sizes of the first and second load resistors 31 and 32, thereby improving the NF characteristics at low frequency (for example, refer to Patent Document 1).
Patent Document 1: Japanese Published Patent Application No. 2003-158425 (Pages 1-6, FIG. 1)
Non-patent Document 1: Nobuyuki Ito, “RF CMOS circuit design technique”, Triceps Corporation, June, 2002, Pages 6-23
However, the above-described methods of improving the low frequency noise characteristics in the mixer circuit have following problems, respectively.
In the first conventional technique, if the LW product is increased, the first and second LO transistors 21 and 22 cannot perform complete switching operations, and thereby the gain is reduced. Further, since the parasitic capacitances of the first and second LO transistors 21 and 22 are increased, the respective frequency characteristics of the RF and LO signals are deteriorated. Therefore, it is not possible to take a so large value for the LW product.
In the second conventional technique, if the W/L ratio of the RF transistor 11 is increased, the distortion characteristics or the frequency characteristics of the RF signal are deteriorated. Therefore, it is not possible to take a so large value for the W/L ratio.
Further, since, as for the bias current, a half of the bias current of the RF transistor becomes the bias currents of the first and second LO transistors 21 and 22, respectively, as is apparent from the circuit construction in FIG. 19, even if gmRF is increased by increasing the bias current, gmLO also increases in proportion thereto, and as a result, it is not possible to obtain a small value for NF.
Though the third conventional technique is effective to some extent when employing a high frequency device such as a bipolar that is superior in the low frequency noise characteristics, it is not so effective when employing MOS devices because the proportion of the flicker noises of the first and second LO transistors 21 and 22 are large. Further, the sizes of the first and second load resistors 31 and 32 are required to be made quite large, and thereby there also arise problems in the circuit area, in the frequency characteristics of the IF signal, and the like.
As described above, the conventional mixer circuits do not have effective measures to reduce the low frequency noises, and particularly, it was not possible to obtain an improved reception sensitivity in a receiving system employing a direct conversion system or a Low-IF system.
The present invention is directed to solving the above described problems, and has for its object to provide a mixer circuit which is superior in the low frequency noise characteristics.